1. Field of the Invention
This invention relates to an organic positive temperature coefficient thermistor that is used as a temperature sensor or overcurrent-protecting element, and has positive temperature coefficient (PTC) of resistivity characteristics that its resistance value increases with increasing temperature.
2. Background Art
An organic positive temperature coefficient thermistor having conductive particles dispersed in a crystalline thermoplastic polymer is well known in the art, as disclosed in U.S. Pat. Nos. 3,243,753 and 3,351,882. The increase in the resistance value is believed to be due to the expansion of the crystalline polymer upon melting, which in turn cleaves a current-carrying path formed by the conductive fine particles.
An organic positive temperature coefficient thermistor can be used as a self-regulating heater, an overcurrent-protecting element, and a temperature sensor. Requirements for these are that the resistance value is sufficiently low at room temperature in a non-operating state, the rate of change between the room-temperature resistance value and the resistance value in operation is sufficiently large, and the resistance value change upon repetitive operations is reduced.
The crystalline thermoplastic polymers used thus far include polyolefins such as polyethylene and polypropylene, polyolefin copolymers of ethylene with various comonomers (e.g., ethylene-vinyl acetate copolymers and ethylenemethacrylic acid copolymers), and fluorine polymers such as polyvinylidene fluoride. Of these, high-density polyethylenes having high crystallinity are often used. The reason is that higher crystallinity polymers have a greater coefficient of expansion and a greater change rate of resistance whereas lower crystallinity polymers have a lower crystallization speed so that when cooled from the fused state, they fail to resume the original crystalline state and exhibit a large change of resistance at room temperature.
One drawback to use of high-density polyethylene is its high operating temperature. The thermistor for use as an overcurrent-protecting element has an operating temperature approximate to its melting point of 130xc2x0 C., which can have a non-negligible thermal influence on other electronic parts on the circuit board. For use as a heat protecting element for a secondary battery, the operating temperature is too high as well. There is a need for a protective element capable of operation at a lower temperature.
Methods for lowering the melting point of polyolefin in order to lower the operating temperature include modifying polyolefin to a structure having many side chains like low-density polyethylene for thereby lowering the density, and introducing comonomers to form copolymers (polyolefin copolymers as mentioned above) for thereby lowering the melting point. Either of these methods, however, results in a polymer with a lower crystallinity, which fails to provide a sufficient resistance change rate or requires a longer time for crystallization. Thus the ability to resume room-temperature resistance upon cooling after operation is substantially impaired.
An object of the invention is to provide an organic positive temperature coefficient thermistor having a lower operating temperature than prior art organic positive temperature coefficient thermistors and exhibiting improved characteristics, and a method for preparing the same.
The inventors have found that the above drawback can be overcome by using a polymer, especially a linear low-density polyethylene (LLDPE), synthesized in the presence of a metallocene catalyst. Specifically, the operating temperature is lowered to about 100xc2x0 C. which is lower than that of high-density polyethylene, while a good resistance resuming ability is maintained. This is accomplished partially because the polymer resulting from polymerization in the presence of a metallocene catalyst has a narrow molecular weight distribution with a reduced content of a low-density, low-molecular weight fraction. Furthermore, prior art LLDPE contains a high-density fraction which crystallizes and serves as crystal nuclei to promote crystallization, whereas the use of a metallocene catalyst ensures uniform creation and growth of crystal nuclei so that even when the polyethylene is melted during operation, the subsequent change of performance is minimized.
According to the invention, conductive particles having spiky protuberances are used in combination, accomplishing both a low room-temperature resistance and a large resistance change rate.
JP-A 5-47503 discloses an organic PTC thermistor comprising a crystalline polymer and conductive particles having spiky protuberances. Also, U.S. Pat. No. 5,378,407 discloses a conductive polymer composition comprising filamentary nickel powder having spiky protuberances, and a polyolefin, olefin copolymer or fluoropolymer. These patent references teach nowhere use of the polymer synthesized in the presence of a metallocene catalyst.
Also, a low-molecular weight organic compound may be further admixed where it is necessary to further lower the operating temperature. In JP-A 11-168005, the inventors proposed an organic PTC thermistor comprising a thermoplastic polymer matrix, a low-molecular weight organic compound, and conductive particles having spiky protuberances. This thermistor has a low room-temperature resistance and a high resistance change rate as well as a lower operating temperature than prior art thermistors using high-density polyethylene matrix. The low-molecular weight organic compound used as an operating substance does not assume the super-cooled state as do polymers, offering a possibility that the transition temperature at which resistance increases upon heating be substantially equal to the temperature at which low resistance is resumed upon cooling.
Where the thermoplastic polymer matrix used in the above-referred patent publication is a low-density polyethylene, the temperature at which the thermistor changes its resistance from high back to low when it cools down after operation is approximately equal to the temperature (operating temperature) at which the thermistor changes its resistance from low to high upon heating (a reduced resistance vs. temperature curve hysteresis). There scarcely occurs the negative temperature coefficient (NTC) of resistivity phenomenon that the resistance decreases after it has once increased. However, the low-density polyethylene has the drawback of a poor ability to resume resistance before and after operation due to its low crystallinity, as previously described.
On the other hand, where the thermoplastic polymer matrix used in the above-referred patent publication is a high-density polyethylene, the ability to resume resistance is good, but there occurs the NTC phenomenon that the resistance decreases after it has once increased during operation at the melting point of the low-molecular weight organic compound, and the temperature at which the thermistor changes its resistance from high back to low when it cools down after operation is higher than the temperature at which the thermistor changes its resistance from low to high upon heating (an increased R-T curve hysteresis).
These problems occur probably because when the low-molecular weight organic compound is melted, its low melt viscosity allows for easy rearrangement of conductive particles so that the resistance decreases after operation or the resistance decreases even at a temperature above the melting point. Where the low-density polyethylene is used as the matrix, its melting point is lower than that of the high-density polyethylene so that when the low-molecular weight organic compound is melted, part of the low-density polyethylene as the matrix is also melted to increase the viscosity of the entire molten components.
This restrains rearrangement of conductive particles, which is the reason why the hysteresis is small and no NTC phenomenon occurs. The NTC phenomenon can trigger the thermal runaway of the thermistor during operation. The increased hysteresis of the latter becomes a problem when the thermistor is used as a temperature sensor such as a heat protecting element. Using a polymer synthesized in the presence of a metallocene catalyst as the matrix, the present invention succeeds in providing an organic PTC thermistor having minimized NTC phenomenon and R-T curve hysteresis and a good ability to resume resistance.
These and other objects are attained by the present invention defined below.
(1) An organic positive temperature coefficient thermistor comprising a polymer synthesized in the presence of a metallocene catalyst and conductive particles having spiky protuberances.
(2) The organic positive temperature coefficient thermistor of (1), wherein said polymer synthesized in the presence of a metallocene catalyst is a linear low-density polyethylene.
(3) The organic positive temperature coefficient thermistor of (1), wherein said conductive particles having spiky protuberances are interconnected in chain-like network.
(4) The organic positive temperature coefficient thermistor of (1), further comprising a low molecular weight organic compound.
(5) A method for preparing an organic positive temperature coefficient thermistor, comprising the steps of
synthesizing a polymer in the presence of a metallocene catalyst,
admixing the polymer with conductive particles having spiky protuberances, and
treating the mixture with a silane coupling agent.
The organic PTC thermistor of the invention is characterized by comprising a polymer synthesized in the presence of a metallocene catalyst and conductive particles having spiky protuberances.
According to the invention, conductive particles having spiky protuberances are used. The spiky shape of protuberances enables a tunnel current to pass readily through the thermistor, and makes it possible to obtain a room temperature resistance lower than would be possible with spherical conductive particles. A greater spacing between protuberant particles than between spherical particles allows for a large resistance change during operation of the thermistor.
The invention also uses a polymer synthesized in the presence of a metallocene catalyst, enabling to lower the operating temperature as compared with prior art organic PTC thermistors. There is obtained a thermistor having improved performance stability despite the low operating temperature, which is difficult to achieve in the prior art. The invention allows a low molecular weight organic compound to be admixed, helping to further lower the operating temperature. The performance stability is improved.